Commonly used apparatuses for measuring the travelling speed of a bicycle consist of a magnet attached to a spoke of the front or rear wheel and a sensor such as a magnetic reed switch or Hall effect sensor attached to the fork or rear part of the frame, for instance to the chain stays or the seat stays. The sensor detects when the magnet passes by as the wheel rotates. The travelling speed is then determined from the distance travelled during a revolution of the wheel, which is equal to the circumference of the wheel, and the time it takes for a full revolution of the wheel, e.g. the time between consecutive passages of the magnet past the sensor. Similarly, the rate at which the bicyclist is pedalling, i.e. the cadence, is typically measured using a magnet mounted on a crank or a chain wheel (also termed front or drive sprocket) and a sensor mounted on the frame, again by sensing passages of the magnet past the sensor, and then determining the number of revolutions of the crankset per unit of time from the elapsed time between consecutive passages of the magnet past the sensor.
Such a prior art solution is described in U.S. Pat. No. 3,898,563 where the use of two separate sensor/magnet arrangements is proposed, one mounted at the front wheel for measuring the travelling speed and the other mounted at the drive sprocket for measuring the cadence.
In order for such a measurement arrangement to function properly it is important that the magnet be exactly aligned with the sensor. Furthermore, the correct operation of this type of measurement arrangement is critically dependent on a careful adjustment of the distance between the magnet and the sensor, since the sensor will only detect the presence of the magnet reliably if it is in close proximity to the sensor. Unfortunately, even when the sensor and magnet are accurately positioned relative to one another during installation of the arrangement on the bicycle, their alignment and the distance between them is very often shifted over the course of time. This can for instance be due to manipulations of the bicycle, e.g. when loading and unloading the bicycle from a car, which frequently requires removing and subsequently reattaching one of the bicycle's wheels, or due to vibrations and impacts acting on the bicycle during its operation. Such displacements between the sensor and the magnet will eventually inhibit the arrangement from functioning correctly, which is especially annoying during a bicycle trip and in particular during a race. Oftentimes, re-adjusting the arrangement requires use of a tool, which may not be at hand whilst riding the bicycle.
A number of solutions have been suggested to alleviate these problems. In U.S. Pat. No. 5,089,775, U.S. Pat. No. 6,188,215 B1 and U.S. Pat. No. 6,331,771 B1 magnet mounting assemblies are proposed which seek to increase the mounting stability of a magnet at a spoke of a bicycle wheel. U.S. Pat. No. 6,957,926 B2 provides a magnet mounting structure which does not require use of a tool to mount or adjust the mounting of a magnet at a spoke of a bicycle wheel. On the other hand EP 1 508 514 A1 and EP 1 923 302 A1 disclose apparatuses including both a speed sensor as well as a cadence sensor to be arranged at a chain stay, thus avoiding having to arrange two separate sensors individually and also seeking to ensure a stable and accurate mounting of both sensors. Furthermore, US 2008/0252038 A1 describes a bicycle frame having a cavity constructed to receive a sensor device, thus allowing to maintain the aerodynamic contour of the frame whilst the sensor device is securely held and accurately positioned in the cavity. The disadvantage of this approach being that by forming a cavity in a frame the stability of the frame structure can potentially be impaired. Moreover, WO 2004/057274 A1 discloses a solution where a magnet and a sensor are integrated into parts of a bicycle. In WO 2004/057274 A1 it is proposed to arrange the magnet of a speedometer internally of a tyre of a bicycle wheel and to securely connect a sensor to the lower end of the steering column, i.e. between the top ends of the two front forks. Further, it is also proposed to arrange a magnet of a pedal revolution counter (for measuring the cadence) on the lateral surface of the spindle within the bottom bracket and to connect the sensor to the frame of the bicycle, e.g. at a bottom end of the seat tube or down tube. With such an arrangement both the magnet and sensor are well protected, but with the disadvantage that both components are poorly or not at all accessible in case repair or replacement is necessary.
Alternative means to measure either the travelling speed or the pedalling cadence of a bicycle are also known. U.S. Pat. No. 4,526,036 discloses a cadence meter comprising means for measuring acceleration intended to be mounted on a non-rotating part of a bicycle. The cadence is determined based on measuring the alternating phases of acceleration and de-acceleration of the bicycle in its direction of travel caused by the changing force applied to the pedals by the bicyclist during each pedalling cycle. A similar technique is employed in the measurement device described in EP 1 213 561 B1. This measurement device includes an accelerometer, which is mounted on a bicycle such that its measurement axis coincides with the direction of travel, the output signal of which is processed in order to extract the frequency of pedalling (i.e. the cadence). Furthermore, WO 2008/058164 A2 presents a crankset based bicycle power measuring device wherein the pedalling cadence is required as part of the power calculation. In conjunction with determining the pedalling cadence, it is mentioned that an accelerometer mounted on a crankset can be used to measure the direction of gravity relative to the orientation of the crank. In WO 2010/000369 A1 the use of an accelerometer in a device for measurement of cycling power output is described, wherein the accelerometer is embedded in a bicycle cleat bolted to a bicyclist shoe or alternatively mounted on a pedal of a bicycle or a leg or foot of a bicyclist. Thereby, the accelerometer is mounted such that its two measurement axes span a plane which is parallel to the two parallel planes of rotation of the cranks. In WO 2010/000369 A1 it is suggested to determine the cadence based on the elapsed time between measuring consecutive minimum and maximum values, respectively, of the output signals from the accelerometer, which occur at the top dead centre and bottom dead centre, respectively, of each revolution of the cranks.